The present invention relates to a pneumatic tire for heavy duty vehicles, more particularly to a tread pattern comprising four circumferential grooves and two circumferential rows of middle blocks, capable of reducing uneven wear occurring on one of the two circumferential rows.
Pneumatic tires fitted onto the drive shafts of a heavy duty vehicle, e.g. truck, bus and the like are required to have high traction performance, therefore, block-type tread patterns made up of blocks in five circumferential rows have been widely employed.
In the case of heavy duty vehicles, during empty running, the loads of the tires fitted onto the drive shaft are decreased to for example 40% of the maximum load for the tire. In other words, when compared with passenger car tires, variations of tire loads are very large. As a result, during empty running, due to the round tread profile, the ground contact pressure is increased in the center block row rather than the axially outermost shoulder block rows and middle block rows. In the middle block rows, accordingly, the amount of slippage against the road surface is increased, and so called punch wear—the middle blocks in one of the two rows wear faster than the other block rows—becomes liable to occur.
Conventionally, in order to reduce punch wear occurring on the middle blocks, so called tie bars are formed between the circumferentially adjacent middle blocks so as to support each other in the tire circumferential direction through the tie bars, to thereby increase the apparent rigidity of the middle blocks and reduce the amount of wear. But, it is difficult for the tie bars to reduce the punch wear till the tire users are fully satisfied. Further, if the volume of a tie bar between the middle blocks is increased, then the apparent rigidity of the middle blocks is increased, and the wear may be reduced, but the transverse groove between the middle blocks is blocked, and as a result, the traction performance is deteriorated.
The present inventor therefore, studied the distribution of wear energy over a middle block and found that, by adjusting the rigidity in each portion of the middle block according to the distribution of wear energy, the rigidity and wear energy are well balanced, and the punch wear of the middle blocks can be minimized without deteriorating the traction performance.